This invention relates to methods of forming of shaped articles of plastics composites comprising fibres dispersed in a matrix of a plastics material.
Composite materials consisting of plastics reinforced by continuous fibres, such as carbon and boron fibres, may be used for making articles having a high strength to weight ratio and desirable properties of stiffness and endurance. Articles made from such composites are increasingly used in the aerospace industry. One type of composite comprises fibres dispersed in a thermoplastic material such as a polyaromatic polyether ether ketone, known as PEEK available from Imperial Chemical Industries plc under the Victrex trade mark. Another type of composite comprises fibre-reinforced thermosetting materials and these are widely used in aerospace and other industries. Composites containing thermoplastic materials have the advantage over composites containing thermosetting plastics that solidification of the plastics material is reversible and a body of the solid composite may be deformed to a desired shape by heating the composite to a temperature at which the plastics material is deformable, followed by forming to shape and cooling. This operation may be repeated and allows articles made of the composite to be repaired; alternatively the composite material can be recycled.
The extent to which a body of fibre/plastics composite may be deformed when hot depends essentially on the arrangement of fibres, which are generally inextensible, in the body. The composites are commonly made as sheets in the form of superposed layers of continuous fibres, in which the fibres of a layer may be woven together and the fibres are impregnated so that individual filaments of the fibre are surrounded and contacted by the plastics material. When the sheet is deformed by bending and/or stretching when hot, the plastics material flows to acquire the new shape of the sheet, but it is also necessary for the structures formed by the fibres to adapt themselves to the new shape. Imperfect adaptation by the fibres can lead to faults of shape and/or unsatisfactory mechanical properties in the shaped article. Some conventional methods of forming cause "buckling" of the fibres which are in compression after deformation, generally on the concave side of a bend; also movement of one layer of fibre relative to the next may produce delamination.
In the case of fibre/thermosettable polymer composites the problem of producing shaped articles containing continuous fibres is less of a problem when the thermosettable polymer is a low viscosity curable resin, because the resin impregnated fibres are flexible and nonrigid intermediate products which can be accommodated in shaped moulds more readily. These systems do however suffer from the disadvantage that long curing cycles are necessary so that the productivity from the moulding equipment is low. The problem of forming reinforced intermediate thermosettable products is however very significant when the thermosettable polymer is not a low viscosity resin but is of sufficiently high viscosity to confer rigidity on the intermediate composite to be shaped and cured. This is the case when the thermosettable material is a solid which has been impregnated into the fibre structure by a solution process, with the solvent subsequently being removed. Such products are stiff, board-like materials which cannot be used in the manner of the low viscosity resin/fibre composite materials to conform to the shape of the mould. The present invention provides a method of forming such board-like intermediate products. The method can also be applied with advantage to the low viscosity resin/fibre intermediate products.